RU2146964C1 - Method of cleaning waste or flue gas containing nitric oxide - Google Patents

Abstract

FIELD: cleaning waste and flue gases. SUBSTANCE: method consists in scrubbing the gas with flushing or scrubbing fluid containing chelate of transition metal. Chelate forms complex with nitric oxide, after which chelate is biologically regenerated in presence of electron donor and nitric oxide is reduced to molecular nitrogen. EFFECT: enhanced efficiency of removal of nitric oxides from waster or flue gases at low capital outlays and operational cost. 12 cl, 5 dwg, 2 ex

Description

 The invention relates to a method for purifying an exhaust or flue gas containing nitrogen oxides, in which the gas is washed with a circulating wash or scrubber liquid containing a transition metal chelate, the chelate being complexed with nitric oxide, nitric oxide reduced to molecular nitrogen, and the chelate essentially regenerate.

 Such a method is described, for example, in Dutch patents 7500672, 7500673, 7515009, 7607212 and 8602001 and European patent 531762. The transition metal chelate, usually iron (II) - EDTA, is used for complexation, therefore, for the effective absorption of nitrogen oxides in which NO very moderately soluble in wash or scrubber water that does not contain a transition metal chelate.

Each of the known methods involves the simultaneous removal of nitrogen oxides, mainly NO and NO ₂ , hereinafter referred to as NO _x and sulfur dioxide, ultimately obtaining molecular nitrogen N ₂ and sulfates or amido sulfates and many other NS compounds , and also, as a rule, N ₂ O. However, the processing of sulfates, N ₂ O and nitrogen-sulfur compounds is usually complicated and requires various subsequent processing and related equipment. N ₂ O will be produced with flue gases. This is an undesirable effect, since N ₂ O is a compound known due to its strong harmful effect on the ozone layer and its strong greenhouse effect.

 Another important problem is that in an oxidizing medium, active Fe (II) is partially converted to less active Fe (III) by means of oxygen from the exhaust or flue gas, or indirectly by means of sulfur in a washing or scrubbing liquid. This results in high chelate losses. In addition, exhaust or flue gases usually contain too little sulfur dioxide (sulfite) compared to nitrogen oxides for the complete regeneration or reduction of the NO-bound Fe (II) -EDTA complex to its active form. Therefore, these methods have not yet found industrial application.

In a method that is already used in practice to remove nitrogen oxides from flue or exhaust gases, the exhaust or flue gas is contacted at a temperature of 300 ^° C. with ammonia NH ₃ and a catalyst to produce nitrogen. This method or process, the so-called selective catalytic reduction process (SCR), however, is expensive due to the high investment associated with the high temperature installation and the high operational cost associated with ammonia and catalyst (approximately one third of the catalyst must be replaced every year) . In addition, a completely separate process is required to optimally remove sulfur dioxide from the same exhaust or flue gas.

The present invention relates to a method that enables the efficient removal of nitrogen oxides from exhaust or flue gases at a significantly lower investment and operating cost, in which the removal of NO _x may optionally be combined with the removal of sulfur dioxide. To the surprise of the authors, it was found that the complex of the transition metal chelate and nitric oxide can be effectively reduced microbiologically to molecular nitrogen and the reduced transition metal chelate. In this method, the transition metal chelate is maintained in a more active state at a lower valency or is returned to a lower valency state.

 The method according to the present invention, as described in the introduction, therefore, is characterized in that the transition metal chelate is regenerated biologically in the presence of an electron donor. In the present description, when mentioning a chelate, it should be understood that a complex of a chelating reagent (or a reagent that causes the formation of chelate compounds) and a transition metal are meant.

 Preferably, the transition metal chelate is Fe (II) - EDTA.

 It is advisable to use methanol, ethanol or other organic matter contained in the liquid stream as an electron donor.

 It is even more advisable to use hydrogen, sulfite or a sulfur-reducing compound other than sulfite, such as sulfide, hydrosulfide, thiosulfate or polythionate.

 Preferably, the regeneration is carried out in the same device in which the exhaust gas is washed.

 It is advisable to use sulfur dioxide present in the exhaust gas as a source of sulfite.

 Preferably, the sulfite or reduced sulfur compound and any sulfate formed are biologically reduced to sulfide, the sulfide obtained is then oxidized to elemental sulfur and the sulfur obtained is then separated.

 Even more preferably, the complex of nitric oxide and transition metal chelate is regenerated in the same apparatus in which sulfite and / or sulfate are reduced.

Biological regeneration according to the present invention, therefore, includes a complex of nitric oxide and a transition metal chelate or a transition metal chelate without nitric oxide. In the first case, nitric oxide is reduced with a concomitant release or release of an active chelate; in the latter case, an inactive chelate in which the transition metal has a higher valency is reduced to an active chelate in which the metal again has a lower valency. The main advantage of this method is that any chelate that has been used in other processes and therefore cannot be suitable for the binding of NO _x is returned to its active form. In principle, the inactive form of the chelate can be restored, for example, by adding a chemical reducing agent or by electrochemical reduction, but in practice this is undesirable due to the higher cost and complexity of the scrubbing cycle (or gas washing cycle).

 A metal such as iron, manganese, zinc, cobalt, nickel or aluminum can be used as a transition metal, which forms a complex with nitric oxide during the formation of a chelate compound. For economic and environmental reasons, iron (II) is preferred, which is in a divalent state in the process of the present invention. A transition metal chelate is formed by a chelating agent that has at least two free electron pairs to form a chelating compound with the metal in the form of amino groups, carboxyl groups or hydroxyl groups. Examples are polyamines, such as, for example, ethylenediamine, diethylenetriamine, triethylenetetraamine, hexamethylenetetramine and 1,4,7-triazonan and their N-alkyl analogues, such as polyamines, for example, ethylenediamine, which contains from one to four hydroxyethyl groups and / or carboxymethyl groups, for example, N- (2-hydroxyethyl) ethylenediamine-triacetic acid and, in particular, ethylenediamine-tetraacetic acid (EDTA), imino-diacetic acid and nitrile-triacetic acid (NTA) and their salts. The concentration of the transition metal chelate may vary in accordance with the characteristic parameters of the gas flushing process. A suitable concentration may be, for example, 1-200 mm, in particular 25-150 mm.

In the method according to the invention, the following reactions occur in which NO is selected as nitric oxide and iron (II) ethylenediaminetetraacetate is selected as an example of a transition metal chelate:
NO + EDTA-Fe ---> NO-EDTA-Fe
NO-Fe-EDTA + [H ₂ ] ---> 1 / 2N ₂ + Fe-EDTA + H ₂ O.

 In this reaction, hydrogen may be molecular hydrogen. Hydrogen can also be present as an electron donor (organic), for example, as methanol, which oxidizes to carbon dioxide under these conditions, or as ethanol. It can also be in the form of another organic substance, which can be chemically oxidized (COD), contained in a stream of liquid (waste).

 Flue gas or flue gas flushing can be carried out in a conventional gas scrubber. The biological regeneration of the complex of the transition metal chelate and nitric oxide can be carried out in the scrubber itself or in a separate bioreactor. The biomass required for biological regeneration contains known nitrate-reducing bacteria.

 A device for cleaning exhaust or flue gas, comprising a gas inlet (2) and a gas outlet (3) for gas scrubber (1), a separator (17) of particulate matter and a connecting pipe (7) between the separator and said scrubber.

 The claimed device is characterized in that it contains at least a first liquid tank (10) and a second liquid tank (13), a connecting pipe (6) between said scrubber (1) and said first liquid tank (10), a connecting pipe (14) between said first and second liquid containers (10) and (13), the separator being connected to said second liquid container (13), wherein the scrubber (1) has an inlet pipe (4) for adding an electron donor and means to ensure effective contact tion of gas and liquid, e.g., the nozzle and the packing material.

 A device for cleaning exhaust or flue gas is also known, comprising a gas inlet (2) for gas and a gas outlet (3) for gas scrubber (1), a particle separator (17) and a connecting pipe (7) between said separator (17) ) and the mentioned scrubber (1).

 The claimed device is characterized in that it contains at least one first liquid container (5) with an inlet pipe (8) for adding an electron donor, an additional second and third liquid container (10) and (13), a connecting pipe (6) between said scrubber (1) and said first liquid container (5), a connecting pipe (19) between said first and second liquid containers (5) and a connecting pipe (14) between said second and third containers (10) ) and (13) for a fluid Op is connected to said third (13) to the liquid reservoir, wherein the scrubber has an inlet conduit (4) for adding an electron donor and a means to provide efficient gas-liquid contacting, e.g., the nozzle and the packing material.

A device for removing NO _x from exhaust gases in which biological regeneration is carried out in a scrubber is shown schematically in FIG. 1. In such a device, the gas is brought into close contact with a washing or scrubbing liquid containing a transition metal chelate and biomass, for example, by means of nozzles and optionally pad material. An electron donor, for example methanol, is added to the wash or scrubber liquid. The resulting nitrogen and any carbon dioxide are removed with purified gas.

 A variant in which biological regeneration is carried out in a separate reactor is shown schematically in FIG. 2. In this device, the washing or scrubbing liquid contains a transition metal chelate, and the used washing liquid is transported to a bioreactor that contains a biomass to which an electron donor is added.

The method according to the present invention can be easily combined with desulfurization, in this case, sulfur dioxide absorbed from the exhaust gas can act as a reducing agent (electron donor). The regeneration should then be carried out in accordance with the reaction given below:
NO-Fe-EDTA + SO ₃ ^2- ---> 1/2 N ₂ Fe-EDTA + SO ₄ ^2- .

 The sulfate formed in this process can be removed by a conventional method (calcium precipitation), but is more preferably biologically removed. Sulfate, possibly with residual sulfite, is therefore reduced anaerobically, mainly to sulfide, and the sulfide formed in this process is then oxidized under limited aerobic conditions to elemental sulfur, which is separated.

The problem with the known method is that the reaction resulting in the formation of molecular nitrogen is only one of several reactions undergoing and often not the main reaction. A product similar to amidesulfates and similar compounds is formed, as well as N ₂ O. These products lead to fouling of off-gas (N ₂ O) and waste water (amide-sulfates). In the method of the invention, these components are also converted to non-hazardous products, and thus unwanted emissions are prevented.

Reduction of NO _x can also be achieved by the presence of other reduced sulfur compounds, such as sulfide, hydrosulfide, sulfur, thiosulfate or polythionate. These sulfur compounds can come directly or indirectly from the exhaust gas or added separately, for example from liquid waste streams.

If sulfur dioxide and other sulfur compounds are used as the reducing agent, the biological regeneration of the transition metal chelate and nitric oxide complex can also be carried out in the scrubber itself or in a separate container. A device for implementing the method in which nitrogen reduction is carried out in a scrubber is shown in FIG. 3. The redox potential in the washing liquid containing biomass in this case is preferably maintained high enough to prevent sulfate reduction, since this can lead to undesirable emission of H _s S. It is preferable to maintain the redox potential above 280 mV, in particular above 200 mV (using an Ag / AgCl reference electrode). The redox potential can be regulated by adding an electron donor.

 Unlike the system of FIG. 1 washing or scrubbing liquid should be additionally treated outside the scrubber in case of reduction with sulfur dioxide to remove the formed sulfate and residual sulfite. This can be done by means of a settling tank (or sump) for the formation of gypsum (not shown). According to a preferred embodiment, the sulfate is treated microbiologically by successive reduction to sulfide in the first vessel (10) and oxidation of the sulfide to elemental sulfur in the second vessel (13), as shown in FIG. 3.

 The same method, but with nitrogen reduction in a separate vessel, can be carried out in the system shown in FIG. 4. In this case, the first tank (5) for reducing nitric oxide, the second tank (10) for reducing sulfate and the third tank (13) for oxidizing sulfide are connected in series downstream of the scrubber.

Nitrogen reduction can also be carried out in one of the sulfur reactors. This embodiment may be implemented in accordance with the system of FIG. 5. NO _x together with sulfate / sulfite can be reduced to nitrogen and sulfide, respectively, through mixed anaerobic biomass. The NO _x residue in the third vessel can also be converted to molecular nitrogen by reaction with sulfide, elemental sulfur and, possibly, thiosulfate. The reduction of NO _x in N ₂ in a third vessel is usually preferable since in this case less electron donor should be added. For this purpose, it may be necessary to reduce the residence time in the third tank so that not all NO _x is fully recovered therein.

 If the gas to be purified contains, in addition to nitrogen oxides, a too low concentration of sulfur dioxide, the amount of sulfur present in the first tank may not be sufficient to completely reduce nitric oxide. Then it will be necessary to add another electron donor (for example, alcohol).

 An important factor that has hindered the use of the Fe chelate up to the present is the oxidation of the active form of Fe (II) to the inactive form of Fe (III) with oxygen from the exhaust gas or with sulfite. In accordance with the present invention, any formed Fe (III) is reduced in the presence of bacteria. A biological reactor can only be used to restore inactive Fe (III) to the active form of Fe (II) and makes the system much more cost effective.

The biological reduction of nitric oxide (in other words, the regeneration of a transition metal complex) is carried out at an approximately neutral pH, for example at a pH in the range from 5 to 9.5, and at elevated temperature, for example, in the range from 25 to 95 ^o C, in particular from 35 up to 70 ^o C.

Description of drawings
FIG. 1 shows a device according to the invention for removing nitrogen oxides in a single scrubber / reactor. In this drawing, the position (1) indicates a scrubber having an inlet pipe (2) for gas and an outlet pipe (3) for gas and means (for example nozzles, packing material) that provide effective gas-liquid contact. In this case, the liquid in the gas scrubber contains a denitrifying biomass. An electron donor may be added via conduit (4).

 FIG. 2 shows a device for removing nitrogen oxides in a separate container. A gas scrubber (1) having an inlet pipe (2) for gas and an outlet pipe (3) for gas and means for providing contact, in this case, is connected to the first container (5), a connecting pipe (6) and a connecting pipe (7) . An electron donor can be added via conduit (8), and gases, mainly nitrogen, can be discharged through conduit (9).

 FIG. 3 shows a device for removing nitrogen oxides and sulfur oxides with denitrification in a scrubber. A gas scrubber (1) having an inlet pipe (2) for gas and an outlet pipe (3) for gas, means for providing contact and an inlet pipe (4) for adding an electron donor, in this case contains a denitrifying biomass and is connected by a pipe (6) with the first tank (10) containing sulfate and sulfite-reducing biomass. An electron donor can be added via conduit (11), and any gases can be discharged through conduit (12) and, if necessary, further processed. The first tank (10) is connected by a connecting pipe (14) to a second tank (13), which contains a biomass oxidizing sulfide and has an inlet pipe (15) for air and an outlet pipe (16) for gas. Downstream, the second vessel (13) is connected to a separator (17) having an outlet pipe (18) for sulfur. The separator (17) is connected by a pipe (7) to a gas scrubber (1) to ensure the return of the wash water.

 FIG. 4 shows a device for removing nitrogen oxides and sulfur oxides with separate denitrification. A gas scrubber (1) having an inlet pipe (2) for gas, an outlet pipe (3) for gas and means for providing contact is connected via a connecting pipe (6) to a first container (5). The first container (5) has an inlet pipe (8) for adding an electron donor and an outlet pipe (9) for gas. Downstream of the first vessel (5), a second vessel (10), a third vessel (13) and a separator (17) are connected, as in FIG. 3.

 FIG. 5 shows a device for removing nitrogen oxides and sulfur oxides with denitrification in an anaerobic sulfur reactor. A gas scrubber (1) having an inlet pipe (2) for gas, an outlet pipe (3) for gas and means for providing contact, in this case, is connected to a second container (10), which has an inlet pipe (11) for an electron donor and an outlet pipe (9) for gas (for example, among other nitrogen) (9). Downstream, the denitrifying sulfate and sulfite reducing second vessel (10) is connected to the third vessel (13), as in FIG. 3.

Example 1
Characteristics of nitrate-reducing bacteria were investigated in a laboratory-scale setup. This setup contains a scrubber and a separate bioreactor. In addition to the Fe-EDTA solution, pure NO was passed through the scrubber, which led to the total conversion of Fe-EDTA to NO-Fe-EDTA. Then, the NO-Fe-EDTA complex was converted in a bioreactor into Fe-EDTA and N ₂ using ethanol as an electron donor. The volume of the bioreactor was 5 dm ³ . After treatment, the liquid was returned to the scrubber, where Fe-EDTA could again be complexed with NO. During the experiments, the temperature was kept constant at 50 ^° C. and the pH was maintained at 7.0. The concentrations of Fe-EDTA used were up to 40 mM. Bacteria converted or converted the NO-Fe-EDTA complex into Fe-EDTA and N ₂ through the following reaction:
6NO-Fe-EDTA + C ₂ H ₅ OH ---> 6 Fe-EDTA + 3N ₂ + 2CO ₂ + 3H ₂ O.

The highest tested load of NO-Fe-EDTA was 5.0 kg of nitrogen / m ³ per day, while it was completely converted by bacteria. The maximum load of NO-Fe-EDTA, which can be controlled by bacteria, has not yet been observed. Toxicity tests have shown that bacteria are not inhibited by Fe-EDTA up to a concentration of 40 mM. From these experiments, it can be expected that higher levels of Fe-EDTA can be used. In addition to regenerating the NO-Fe-EDTA complex, bacteria have found the ability to restore inactive Fe (III) -EDTA to active Fe (II) -EDTA. Fe (III) -EDTA is formed due to the reaction of Fe (II) -EDTA with oxygen present in the exhaust gas and the reaction of NO-Fe-EDTA with sulfite. In the corresponding experiments, instead of NO, air was passed through the reactor, which led to the complete oxidation of Fe (II) -EDTA to Fe (III) -EDTA. Then Fe (II) -EDTA was regenerated in the bioreactor. At a concentration of inflowing Fe (III) -EDTA of 5 mM and a hydraulic retention time of 1.5 hours in the bioreactor, the bacteria showed complete recovery to Fe (II) -EDTA. Higher concentrations of Fe (III) -EDTA were not used.

Example 2
Exhaust gas flowing at a flow rate of 45,000 m ³ / h and containing 670 mg / m ³ SO ₂ (250 ppm or ppm v / v) and 1670 mg / m ³ NO _x (expressed in NO and containing 5-20 % NO ₂ ) (1340 ppm v / v) was processed in the off-gas purifier shown in FIG. 5. The volume of the scrubber was 70 m ³ and the flow rate of the washing liquid was 600 m ³ / h. The volume of the anaerobic reactor was 275 m ³ and the volume of the aerobic reactor was 45 m ³ . The circulation flow through the bioreactor was 110 m ³ / h. Wash or scrubber water contained 3 g of Fe-EDTA per 1. The SO _x removal efficiency was 99%, and the NO _x removal efficiency was 75-80%.

Claims (12)

 1. The method of purification of exhaust or flue gas containing nitrogen oxides, in which the gas is washed with a circulating wash or scrubber liquid containing a transition metal chelate, the chelate complexing with nitric oxide, then the chelate is regenerated and nitrogen oxide is reduced to molecular nitrogen, characterized in that the transition metal chelate is regenerated biologically in the presence of an electron donor.  2. The method according to claim 1, characterized in that the transition metal chelate is Fe (II) -EDTA.  3. The method according to claim 1 or 2, characterized in that methanol, ethanol or other organic matter contained in the liquid stream is used as an electron donor.  4. The method according to claim 1 or 2, characterized in that hydrogen is used as an electron donor.  5. The method according to any one of claims 1 to 4, characterized in that the regeneration is carried out in the same device in which the exhaust gas is washed.  6. The method according to claim 1 or 2, characterized in that sulfite is used as an electron donor.  7. The method according to claim 6, characterized in that as the source of sulfite use sulfur dioxide present in the exhaust gas.  8. The method according to claim 1 or 2, characterized in that as the electron donor use a sulfur-reducing compound other than sulfite such as sulfide, hydrosulfide, thiosulfate or polythionate.  9. The method according to any one of paragraphs. 6 to 8, characterized in that the sulfite or reduced sulfur compound and any sulfate formed are biologically reduced to sulfide, the resulting sulfide is then oxidized to elemental sulfur and the sulfur obtained is then separated.  10. The method according to claim 9, characterized in that the regeneration of the complex of nitric oxide and a transition metal chelate is carried out in the same device in which sulfite and / or sulfate are reduced.  11. A device for cleaning the exhaust or flue gas, comprising a gas inlet (2) and a gas outlet (3) for a gas scrubber (1), a particulate separator (17) and a connecting pipe (7) between the separator and said scrubber characterized in that it contains at least a first liquid container (10) and a second liquid container (13), a connecting pipe (6) between said scrubber (1) and said first liquid container (10), the pipeline (14) between the aforementioned first and watts with liquid containers (10) and (13), the separator being connected to said second liquid container (13), wherein the scrubber (1) has an inlet pipe (4) for adding an electron donor and means for ensuring effective gas and liquid contact for example nozzles and packing material.  12. A device for cleaning the exhaust or flue gas, comprising a gas inlet (2) and a gas outlet (3) for a gas scrubber (1), a particulate separator (17) and a connecting pipe (7) between said separator (17) ) and the said scrubber (1), characterized in that it contains at least one first liquid container (5) with an inlet pipe (8) for adding an electron donor, additional second and third containers (10) and (13) for liquid, a connecting pipe (8) between said scrubbero m (1) and said first liquid container (5), a connecting pipe (19) between said first and second liquid containers (5) and (10) and a connecting pipe (14) between said second and third containers (10) and (13) for a liquid, the separator being connected to said third (13) liquid container, the scrubber having an inlet conduit (4) for adding an electron donor and means for ensuring effective contact of the gas and the liquid, for example, nozzles and packing material.

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